This invention relates to an improved method for the treatment of ores to enhance the recovery of metals by hydrometallurgical processes and, in particular, to the recovery of silver from manganese-silver ores by reduction roasting followed by cyanide leaching.
The recovery of metals from ores is an important industrial process and is practiced following a number of diverse procedures depending mainly on the nature of the ore. In many cases extraction of the metal is obtained by concentrating the ores followed by pyrometallurgical techniques in which intense heat is used to convert the metal mineral into the metal. In other extraction processes the desired metal is preferentially leached from the ore and then recovered from the leachant solution. Many of the ores are not directly leachable or are refractory however, and they must first be treated to increase the leachability of the metals. Throughout this specification and claims, the term "ore" is intended to include concentrates, tailings and other metalliferous matter as well as raw ores.
A particular well-known process is the treatment of manganese containing ores to increase the recovery of manganese by acid dissolution and the treatment of manganese-precious metal refractory ores to increase the recovery of the precious metal, e.g., silver, by cyanide leaching of the reduced ore and, for convenience, the following description will be directed to manganese containing ores but it will be appreciated by those skilled in the art that other ores may be similarly treated.
Regarding the recovery of silver from manganese-silver refractory ores, this process has presented many problems to the art over the years. Early work was performed by G. H. Clevenger and M. H. Caron and summarized in "The Treatment of Manganese-Silver Ores" published by the Department of the Interior, Bureau of Mines, 1925, Bulletin 226. U.S. Pat. No. 1,232,216 issued to Caron discloses the need for reduction roasting of the ore before cyanidation. As theorized by Caron, the low silver extraction is due to the presence of manganese dioxide in contact with the silver particles preventing normal action of the cyanide solution on the silver particles. Caron shows reduction of the ore using CO, or other reductants. A summary entitled "Treatment of Manganiferous Silver Ores for Recovery of Silver in View of Changed Precious Metal Ecomonics" by C. K. Chase is presented in the Proceedings from the 110th AIME Meeting in Chicago, Ill., Feb. 22-26, 1981. U.S. Pat. No. 1,562,942 shows roasting of the ore with pyrite or with other sulfides or sulfur. U.S. Pat. No. 1,864,222 utilizes solid carbon as the reductant and U.S. Pat. No. 2,759,809 reduces the ore by heating cellulosic materials such as wood with the ore in the absence of air. U.S. Pat. No. 2,867,529 has a preliminary roasting operation in an inert atmosphere or in an atmosphere deficient in oxygen followed by oxidation in an aqueous acid solution. A process for the direct cooling of reduced manganese ore without significant reoxidation is disclosed in U.S. Pat. No. 3,932,945.
Other alternatives to reduction roasting are hydrometallurgical in nature as shown in U.S. Pat. Nos. 2,829,045; 2,839,387; 2,884,322 and 4,421,724 and basically relate to extracting the ore with chemicals such as nitrites, amines and organic phosphoric acids either alone or in combination with cyanidation. U.S. Pat. Nos. 1,327,974 and 1,417,153 disclose mixing pulverized ore and pyrite or iron with H.sub.2 SO.sub.4 to reduce the manganese. U.S. Pat. No. 2,777,764 shows a precyanidation treatment procedure utilizing a nonalkaline oxidizing medium such as H.sub.2 SO.sub.4 under pressure at high temperature and T104,001 shows oxidizing comminuted ore under pressure at an elevated temperature.
The separation of manganese from manganese dioxide containing ores has also presented similar problems. In general, the ore is first reduced from MnO.sub.2 to MnO with reducing agents such as pulverized coal or heavy oil in rotary kilns or with natural gas or hydrogen by pile roasting at about 700-900.degree. C. After the reduction, the ore is cooled in a nonoxidizing atmosphere , e.g., inert or reducing, to prevent spontaneous reoxidation of the MnO and is then leached or dissolved with H.sub.2 SO.sub.4 to separate the manganese for purification.
The above patents and references, the disclosures of which are hereby incorporated by reference, still do not provide an efficient method for the treatment of ores and in particular to the enhanced recovery of manganese from manganese dioxide ores or to the recovery of silver from refractory ores however, and the need exists for an improved process to provide high yields.